That is a Thagomizer. Do you know why it’s called that? If you said it’s called that because of a guy named Thag, you’re half right. If you said it’s called that because of the late Thag Simmons, you’re awesome.

Because this:

is where the term originated. A Far Side comic. And yes, it has become since then, a regular anatomical term. Mainly because of one palaeontologist, who also like Gary Larson!